1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming capable of effectively reducing unevenness of density and color displacement of images by controlling a speed of a transfer member with a high precision.
2. Discussion of the Background
Conventionally, image forming apparatuses are widely used and known for electro-photo graphic copiers, printers, and facsimiles, or for composite machines having at least one functional combination among electro-photo graphic copier, printer, and facsimile.
For example, an image forming apparatus using a direct transfer method or an intermediate transfer method are widely known.
In the direct transfer method, toner images of each color formed on a plurality of image carrying members are overlayingly transferred to a recording sheet carried and transported on a sheet transport belt. The sheet transport belt is extended to a plurality of support rollers, and at least one of the support rollers is used as a drive roller. The sheet transport belt is driven by the drive motor.
In the intermediate transfer method, toner images of each color formed on a plurality of image carrying members are overlayingly transferred to an intermediate transfer belt, and are transferred to a recording sheet. The intermediate transfer belt is extended to a plurality of support rollers, and at least one of the support rollers is used as a drive roller. The intermediate transfer belt is driven by the drive motor.
A typical image forming apparatus using these methods is provided with a plurality of image carrying members arranged in series, which is referred to as an image forming apparatus of tandem type.
In such an image forming apparatus, it is important not to cause an unevenness of density in a toner image transferred from an image carrying member to a recording sheet or a to an intermediate transfer belt to improve an image quality of a finished image transferred to the recording sheet. To prevent the unevenness of density in the toner image, the image forming apparatus includes a detection roller, a moving member, and a sensor.
At least one of a plurality of support rollers for a sheet transport belt or an intermediate transfer belt is used as the detection roller. The moving member is provided to the detection roller, and rotates with the detection roller. A moving speed of the moving member is detected by the sensor.
Based on a detection result, the drive motor is controlled to make a traveling speed of the sheet transport belt or the intermediate transfer belt to a target speed in a conventional apparatus.
With such a configuration, the traveling speed of the sheet transport belt or the intermediate transfer belt is stabilized not to cause the unevenness of density in the toner image transferred to the recording sheet carried and transported on the sheet transport belt, or not to cause the unevenness of density in the toner image transferred to the intermediate transfer belt from the image carrying member.
Conventionally, a moving speed of the moving member is detected by the sensor with a following configuration.
The detection roller has a roller shaft coupled to a support shaft through a coupling, and the moving member is fixed to the support shaft.
The rotation of the detection roller is transmitted to the support shaft through the roller shaft and the coupling to rotate the moving member, and the moving speed of the moving member is detected by the sensor.
In this case, when the support shaft has an eccentricity against the roller shaft, such an eccentricity causes a cyclical variation in speed of the moving member per rotation, and such a variation in speed of the moving member is detected by the sensor along with other variations in speed. Then, a drive motor is controlled to eliminate all of the variations in speed.
However, the variation in speed of the moving member caused by an eccentricity of the support shaft against the roller shaft is not caused by the variation of the traveling speed of the sheet transport belt or the intermediate transfer belt.
Therefore, if the variation in speed of the moving member caused by an eccentricity of the support shaft is eliminated by a control of the drive motor, the variation in speed of the transfer belt inadvertently occurs, thus the unevenness of density occurs in the toner image transferred to the recording sheet from the image carrying member or from the intermediate transfer belt.
Conventionally, a detecting mechanism is configured as illustrated in FIG. 1.
The detecting mechanism includes a detection roller 10, a roller shaft 42, a slit disk 43, a photo sensor 44, a control device 45, a coupling 60, a support shaft 61, and a drive motor 13.
The detection roller 10 is supported by the roller shaft 42. The roller shaft 42 is coupled to the support shaft 61 through the coupling 60. The slit disk 43 as a moving member is fixed to the support shaft 61. The roller shaft 42 rotates with the detection roller 10. The rotation of the roller shaft 42 is transmitted to the support shaft 61 through the coupling 60 to rotate the slit disk 43. The moving speed of the slit disk 43 is detected by the photo sensor 44, and the drive motor 13 is controlled based on the detection results.
However, the above-described detecting mechanism has drawbacks as follows.
As for the detecting mechanism of FIG. 1, the detection roller 10 is concentrically coupled to the roller shaft 42, and the support shaft 61 is also concentrically coupled to the roller shaft 42. Furthermore, the slit disk 43 is concentrically coupled to the support shaft 61.
However, it is difficult to couple all of these components concentrically with each other.
As for the detecting mechanism of FIG. 1, the detection roller 10 has a slight eccentricity δ1 against a central axis line X1 of the roller shaft 42.
The slit disk 43 also has a slight eccentricity δ2 against a central axis line X2 of the support shaft 61.
Furthermore, the central axis line X2 of the support shaft 61 has inevitably a slight eccentricity δ3 against the central axis line X1 of the roller shaft 42.
Such eccentricities cause variations in a speed for each rotation of the slit disk 43. The variation in a speed of the slit disk 43 is detected by the photo sensor 44, and the drive motor 13 is controlled to eliminate the variation in a speed.
However, when the detection roller 10 is a driven roller, and the drive motor 13 is controlled to eliminate the variation in speed caused by the above-described eccentricities, a variation in traveling speed inadvertently occurs to the sheet transport belt or the intermediate transfer belt.
If such variations in traveling speed of the sheet transport belt or the intermediate transfer belt are not corrected, a color displacement occurs in a synthesized toner image transferred to the recording sheet from the image carrying members or the intermediate transfer belt.
The variation in speed of the slit disk 43 caused by the eccentricities of the detection roller 10, the slit disk 43, and the support shaft 61 is erroneously detected as the variations in traveling speed of the sheet transport belt or the intermediate transfer belt, although the variations in traveling speed of the sheet transport belt or the intermediate transfer belt do not actually occur. When the traveling speed of the sheet transport belt or the intermediate transfer belt is controlled based on such erroneous detection, a color displacement occurs on a finished color image.
Similarly, when the detection roller is a drive roller, the variation in speed of the slit disk 43 caused by the eccentricities of the slit disk 43, and the support shaft 61 is erroneously detected as the variations in traveling speed of the sheet transport belt or the intermediate transfer belt. When the traveling speeds of the sheet transport belt or the intermediate transfer belt are controlled based on such erroneous detection, a color displacement occurs on a finished color image.
The above-described configuration is not intended to eliminate the variation in speed of the slit disk 43 caused by the above-mentioned eccentricities.
Therefore, the above-described configuration cannot prevent the unevenness of density of each toner image transferred to the recording sheet from each of the image carrying members or the intermediate transfer belt. Such unevenness of density becomes more significant when the above-mentioned eccentricities are accumulated.
When the support shaft 61 is coupled to the roller shaft 42 through the coupling 60, and the slit disk 43 is fixed to the support shaft 61 as illustrated in FIG. 1, the eccentricity δ3 of the support shaft 61 against the roller shaft 42 is accumulated to other eccentricities δ1 and δ2, thus the total eccentricity becomes larger. Consequently, the unevenness of density is likely to occur in the toner image transferred to the recording sheet from the image carrying members or the intermediate transfer belt.
In addition, when a plurality of color images are not overlayed correctly in an image forming apparatus a color displacement will occur, and degrade an image quality.
Several factors are critical for a correct overlaying of the plurality of color images. The most important factor is variations of the traveling speed of a belt such as a sheet transport belt or an intermediate transfer belt. Suppressing the variation in the traveling speed of the belt is critical to improve precision for overlaying the plurality of color images, however, the variation in the traveling speed of the belt occurs by several causes.
For example, with the demand of miniaturization of an apparatus, dimensions between each unit in the image forming apparatus becoming smaller. Therefore, variation in the traveling speed of the belt such as a sheet transport belt or an intermediate transfer belt may occur with an influence of the heat generated at a heat source such as a fixing device.
To eliminate such heat influence, providing heat pipes to rollers extending the belt to indirectly cool the belt is proposed. However, such a solution leads to a complication of the configuration, and also to an increase of the manufacturing cost of the image forming apparatus.
Furthermore, a sensor to detect a traveling speed of the belt may be provided to stabilize the traveling speed of the belt based on an output signal from the sensor. However, the traveling speed of the belt may not be detected correctly by the heat expansion of the sensor.